Pluto is a dwarf planet at the outer edge of our solar system, about 3.6 billion miles from the Sun. For 76 years, from its discovery in 1930 until 2006, astronomers classified Pluto as the ninth planet. Then scientists realized that Pluto shares its neighborhood with many other icy objects in a region called the Kuiper Belt, so they reclassified it as a dwarf planet.

This decision sparked passionate debate. Many people felt attached to Pluto as a planet, having grown up learning about the “nine planets.” Pluto is smaller than Earth's moon and has an unusual, tilted orbit that sometimes brings it closer to the Sun than Neptune.

In 2015, NASA's New Horizons spacecraft flew past Pluto and sent back stunning photographs showing mountains of frozen water ice and plains of frozen nitrogen. The images revealed a world far more complex and beautiful than anyone expected. Pluto has five known moons, the largest being Charon, which is so big relative to Pluto that the two orbit each other like partners dancing.

Whether you call it the ninth planet or the most famous dwarf planet, Pluto remains a fascinating world at the frontier of our solar system, showing that exploration can lead to new understanding, even when that means revising what people thought they knew.